CHILLICOTHE (AP) — Prison guards and nurses were among two dozen people treated for possible drug exposure inside an Ohio prison Wednesday, the state highway patrol said.

The individuals, who included one inmate, started showing symptoms of an overdose around 9 a.m. at Ross Correctional Institution, said patrol spokesman Lt. Robert Sellers. They were administered naloxone, a drug used to combat overdoses caused by opioids such as heroin or fentanyl. The prison is about 44 miles (71 kilometers) south of Columbus.

Sellers said the patrol made 300 doses of naloxone available if needed. Contraband fentanyl, a powerful synthetic opioid, is suspected, possibly dispersed into the air by a fan, said Sellers, who emphasized investigators were still trying to determine what happened.

Inmates were evacuated from the unit where the exposure occurred, and a hazmat team from the Columbus fire department was summoned to clean up the affected area. Messages were left for the prisons system and the union representing correctional officers. Contraband drugs including opioids are an ongoing issue for the Department of Rehabilitation and Correction and Ohio jails.

In May, a former milk deliveryman was accused of hiding marijuana, tobacco and cellphones inside milk cartons and smuggling them into a prison in southwest Ohio. He was sentenced to house arrest.

In September 2017, the Richland County Jail in Mansfield started operating a body scanner to detect drugs. Officials there monitored 26 opioid-related withdrawals in July 2017. In February 2017, four inmates overdosed in two days at Pickaway Correctional Institution south of Columbus, requiring the use of CPR and doses of an anti-overdose drug as guards scrambled to revive the men.